The National Weather Service predicted Monday that more than 90 U.S. temperature records will be tied or broken this week through Wednesday, and that most of them will be overnight records rather than daytime highs [1]. A long-lasting heat dome is expected to blanket much of the country, and the danger is the temperature the thermometer refuses to fall to after dark.

The specific numbers are unusual for the places attached to them. Over the coming days, cities known for frigid winters are forecast to hold nighttime temperatures above 70 degrees Fahrenheit, including Fargo, North Dakota; International Falls, Minnesota; and Portland, Maine, according to the weather service [1]. Along the Gulf and Southeast coasts, the floor is higher still: temperatures were not forecast to drop below 80 degrees at night in Fort Lauderdale, Miami, Tampa, Galveston, Texas, and Charleston, South Carolina [1]. Minneapolis sat under an extreme heat warning Monday, with residents diving into Cedar Lake and paddleboarding on Lake of the Isles to escape it [1].

That is the distinction weather feeds tend to flatten. Daytime highs and red map colors dominate the forecast graphic, but the AP report centers the quieter danger: a night that stays above 70 or 80 degrees denies the body its recovery window. Health experts told the AP that overnight temperatures which fail to cool are more dangerous than the daytime peaks that make headlines, because the body's core temperature never gets the chance to fall and recover from the day's load [1]. A gain of just a few degrees in body temperature can trigger heatstroke or overstrain the heart.

The consequence is cumulative, and it is delayed. "Mortality starts the second or third day" because the body is unable to cool, Kristie Ebi, a public health and climate scientist at the University of Washington, said Monday [1]. That lag is what the record count obscures. Ninety broken records is a single-week statistic; the deaths it foreshadows arrive later and land unevenly. "That's where the health outcomes are amplified, particularly for the elderly and vulnerable communities," said Marshall Shepherd, a meteorology professor at the University of Georgia [1].

The weather service framed the event as a forecast, not an accounting. Joe Wegman, a meteorologist at the service's Weather Prediction Center, told the AP in an interview that a heat wave should be taken seriously [1]. What the service issued Monday is a prediction of records that could fall through Wednesday, and an official warning to treat the nights as the hazard. It is not yet a count of who was harmed. Early warning signs include heavy sweating, muscle cramps and headache; the practical advice the AP relays is narrow and physical, such as stepping into air conditioning or wrapping a cold towel around the neck.

The gap between the two frames costs the reader something specific. The frame that circulates on weather feeds emphasizes the visible spectacle: the heat dome map, the triple-digit afternoon high, the swimmers at the lake. The AP report emphasizes the invisible variable, the overnight low, and attributes the danger to human-caused climate change, which experts told the service is making heat waves last longer and grow more intense [1]. A reader who tracks only the afternoon high will read the peak and relax at sunset. The distance between those two frames is measured in the second and third nights, when the mortality Ebi described begins.

The count of 90-plus records is a forecast, not a tally of harm. How many overnight records were actually observed rather than predicted, how many cooling centers opened, how many outages cut power to the vulnerable, and how many heat illnesses or deaths followed will not be known until the days after the records fall. The toll of who could not cool down arrives last.

-- NORA WHITFIELD, Chicago